1. Technical Field
The present invention relates to a memory device and a signal driving device thereof.
2. Background
A signal driver for memory cells provides a larger signal at low supply voltage (VCC). Usually, memory data is stored as small and weak signals and thus needs to be buffered by a signal driver and delivered to periphery circuitry in order to be read from a chip. A stronger signal driver increases the signal margin. However, there is a drawback to a stronger signal driver. Since the signal driver has to be placed repeatedly in a memory core, which is the most expensive area in terms of space in the memory chip, enlarging the driver size has a direct impact on chip size and thus on chip cost. Additionally, signal margin is usually too small for low VCC situations.
FIG. 1 shows a conventional signal driver 10. Referring to FIG. 1, the conventional signal driver 10 includes a constant current circuit 11 and a bias circuit 12. The constant current circuit 11 includes a constant current source 110 and a sixth MOS transistor 111. The bias circuit 12 includes a first switch MOS transistor 101, a second switch MOS transistor 102, a third switch MOS transistor 103, a fourth switch MOS transistor 104 and a bias MOS transistor 105. The constant current source 110 provides a constant current Iref. A voltage LBias is provided by the sixth MOS transistor 111 for the gate 109 of the fifth MOS transistor 105. The bias voltage LBias controls the fifth MOS transistor 105 to provide a driving current IOD. A signal Lio is applied to the gate 106 of the first switch MOS transistor 101 and a signal LioF is applied to the gate 107 of the second switch MOS transistor 102, and both are launched onto global IO lines Gio/GioF to be read out to periphery circuitry. The third switch MOS transistor 103 and the fourth MOS transistor 104 are both switch controlled by Rsel 108 as the function of a read select signal. The fifth MOS transistor 105 controls the current IOD under the bias voltage LBias 109. Because the resistances of the switch MOS transistors 107 and 108 are very sensitive, the switch MOS transistors 107 and 108 should be large enough to avoid an excessive current-resistance drop at low VCC. However, the large switch MOS transistors 107 and 108 enlarge the driver size, causing a direct impact on chip size and chip cost.